1. Technical Field
The present invention relates to a gas generator for a vehicle occupant restraint, such as an air bag, and to a gas generating composition which is used in the gas generator.
2. Description of the Prior Art
A large number of gas generating compositions have been proposed for generating gas to operate vehicle occupant restraints, such as air bags and pretensioners for seat belts. For air bags, in particular, compositions which produce an inert gas, such as nitrogen, have been preferred.
Excellent results have been achieved with solid nitrogen generating compositions which comprise an alkali metal azide fuel and one or more oxidizing agents for the fuel. Compositions comprising these materials produce, on combustion, a nitrogen gas along with other products of reaction.
One preferred oxidizing agent for an azide containing gas generating material is a metal oxide. A particularly preferred metal oxide is ferric oxide (Fe.sub.2 O.sub.3). Ferric oxide is preferred because it is commercially readily available, less expensive than other metal oxides, and easier to work with when manufacturing gas generating material.
The form of ferric oxide which has been traditionally used, in combination with sodium azide, in the preparation of a gas generating composition for a vehicle occupant restraint, is alpha iron oxide. U.S. Pat. No. 4,902,036 discloses that this ferric oxide, though capable of a relatively high effective gas output, often requires the addition of a burn rate enhancer for a gas generating composition containing the oxidizer to reach its full potential.
U.S. Pat. No. 4,604,151 discloses data comparing the burning rate and heat of reaction of a sodium azide containing gas generating composition, in which the oxidizer is ferric oxide, with the same or a similar composition in which the oxidizer is nickel oxide (NiO.sub.2) or copper oxide (CuO). Specifically, a composition having 65.5% sodium azide, 30% ferric oxide, and 4.5% ammonium perchlorate had a heat of reaction of 410 calories/gram and a burning rate, at 1,000 psi, of 1.30 in./sec. In contrast, a similar composition containing 30% nickel oxide had a heat of reaction of 456 calories/gram and a burning rate, at 1,000 psi, of 1.46 in./sec. A composition containing 61% sodium azide, 34.5% copper oxide, and 4.5% ammonium perchlorate, had a heat of reaction of 487.8 calories/gram and a burning rate, at 1,000 psi, of 2.24 in./sec.
U.S. Pat. No. 4,243,443 also discloses the reaction of ferric oxide with sodium azide. This patent specifically identifies .alpha.-Fe.sub.2 O.sub.3 as the iron oxide which was used. The patent discloses that one problem with this iron oxide is the reproducibility of burn characteristics from test to test. The patent proposes doping the ferric oxide with nickel, and discloses data showing a substantial improvement in burn rate of a composition containing a nickel doped ferric oxide over a composition containing an undoped ferric oxide.
U.S. Pat. No. 4,698,107 discloses applying an ignition enhancing coating to grains or pellets of a gas generating composition, which contains an alkali metal azide and ferric oxide. The coating contains ingredients which ensure a reliable ignition of the coating by an igniter. The burning of the ingredients of the coating provides a heat transfer to ignite the material of the gas generating grains or pellets.
Other ignition enhancing coatings are disclosed in U.S. Pat. Nos. 4,244,758, 4,246,051, and 4,390,380.